1. Field of the Invention
The present invention relates to a method for recovering carboxylic acids from dilute aqueous streams by extraction with high-boiling organic acid or ester solvents and concentration. More particularly, the present invention relates to a method for recovering acrylic acid from dilute acid water streams in processes for the manufacture of acrolein or acrylic acid.
2. Discussion of the Prior Art
Both acrolein and acrylic Acid (AA) are conventionally produced by gas-phase catalytic oxidation of propylene. There are also reports in the patent and open literature that with suitable catalysts, propane can be used as a feedstock lieu of propylene.
In acrolein manufacture, the reaction is typically carried out in a single-stage reactor, optimized to selectively oxidize propylene to acrolein, with a minimum of byproducts. However, some over-oxidation occurs resulting in the production of AA as well. In AA manufacture, the reaction is typically carried out in two stages, oxidizing propylene to acrolein in the first stage (as in acrolein manufacture), and then further oxidizing the acrolein to AA in the second stage.
In both acrolein and AA manufacture, AA is first separated from the gas phase reactor effluent by absorption into water, resulting in a dilute aqueous AA stream that also contains water-soluble, medium- and higher-boiling reaction byproduct impurities such as acetone, allyl alcohol, acetic acid, propionic acid, and maleic acid.
In AA manufacture, the aqueous AA stream leaving the absorber typically contains less than 40-65% AA. This crude aqueous AA stream is sent to a purification system, which typically involves a series of energy-intensive distillation columns.
Because of the relatively high water content, and the fact that AA forms azeotropes with water and other reaction impurities, the AA purification system is complex and energy intensive.
In acrolein manufacture, the aqueous AA stream leaving the absorber typically contains less than 10% AA; at stand-alone manufacturing sites, it is handled as a waste. Although dilute in AA, the concentrations are sufficiently high to make it impractical to treat the wastewater by relatively low-cost means such as biological treatment or wet-air oxidation. Thus, the AA wastewater is typically sent to an incinerator. Operating the incinerator requires a large amount of fuel owing to the large amount of water present, relative to the AA. Thus, handling and incineration of the dilute AA stream represents a significant operating expense in the manufacture of acrolein. While in principle the aqueous AA could be transported to an off-site AA manufacturing facility for recovery of the AA, the dilute AA concentration makes transportation costs prohibitive.
In the case of acrolein manufacture, it would be desirable to separate and recover AA from the dilute wastewater stream, yielding an AA concentrate, and an AA-depleted wastewater. The AA concentrated could be transported economically to an off-site AA manufacturing facility, for purification of the AA into a commercially valuable product. The AA-depleted wastewater may have a sufficiently low concentration of residual organic compounds, so that it can be feasibly treated by less expensive means than incineration, e.g. biological wastewater treatment or wet-air oxidation.
In case of AA manufacture, it would be desirable to separate the AA from the water in crude AA exiting the absorber. Reducing the water loading in the crude AA sent to purification reduces the energy requirements and costs in the distillation train.
The prior art describes various extraction-based processes for separating carboxylic acids, especially AA, from aqueous streams. However, most of these involve solvents that boil relatively close to or below the boiling point of the carboxylic acids of interest, especially AA, and are primarily intended for use with concentrated aqueous carboxylic acid streams. Separation of the carboxylic acid from the solvents generally requires complex and energy-intensive distillations. As the typical solvents generally are boiled overhead, the carboxylic acids are taken as the bottoms streams, exposing them to high temperatures that can degrade the product.
In order to avoid taking the carboxylic acids as bottom streams, the alternative is to use solvents which are less volatile, i.e. high-boiling, than the carboxylic acids, as the carboxylic acids are boiled overhead, and the solvent generally is taken as the bottoms streams. The fouling tendency can be further mitigated by reducing the pressure of the distillations, and/or the addition of polymerization inhibitors to the distillation system. This is well known in the art.
Prior art related to Acrylic Acid separation using low-volatility solvents includes the following:
U.S. Pat. No. 3,534,091 discloses the use of C6-C22 fatty carboxylic acids as extraction solvents for separating C3 and C4 carboxylic acids, including AA, from aqueous solutions. It describes the use of isooctanoic acid (i.e. 2-methyl heptanoic acid), among others, as the solvents. Example 5 teaches the use of isooctanoic acid to extract AA from a 1.135 wt % aqueous solution, with a 1:9 solvent:aqueous volumetric ratio. Example 1 used a 1:1 solvent:aqueous feed ratio, where the aqueous contained 0.864 wt % AA, and achieved an AA recovery of 54% to the extract.
U.S. Pat. Nos. 6,166,248 and 6,555,707 disclose the use of solvents such as lactams (methylpyrrolidone) or other organic acids such as 2-ethylhexanoic acid as an absorption solvent to remove AA from hot gaseous streams. The gas stream containing non-absorbed components is condensed into an acid water stream which is generally incinerated.
U.S. Pat. No. 6,281,386 discloses of the use of high boiling solvents, specifically claiming heavy solvents that are aromatic compounds with normal boiling points between 260° and 380° C. in the purification of acrylic acid.